Insulated concrete forms (ICFs) all share a design that combines insulation and concrete to form finished walls. Some types are made with 2 inches of closed-cell extruded polystyrene on both sides of the wall, with spacers in between to hold the form together as the concrete is poured. The spacers do double duty by serving as screw bases for attaching finish wall materials on the inside and outside, to cover the foam. Some ICFs are stacked like blocks, and their internal cavities filled with concrete. In all cases, there is insulation on both sides of the foundation wall that keeps the basement warm and helps make the wall more water-resistant. The exterior surface in contact with the ground should still be coated with a waterproof membrane.